1. Field of the Invention
This invention relates to a method of migrating an alloy melt of a carrier metal and passenger element elected for its doping characteristics through a solid body of semiconductor material by thermal gradient zone melting (TGZM) and, in particular, to the case where the carrier metal and its passenger element react to form an inert stable compound that make the combination of the particular carrier metal and passenger element generally unsuitable for TGZM.
2. Description of the Prior Art
W. G. Pfann in U.S. Pat. No. 2,739,088 and 2,813,048 describes methods of migrating melts of metal through particular regions of a solid body of semiconductor material by thermal gradient zone melting.
Although aluminum has been the predominant metal migrated by TGZM, one needs configurations other than P.sup.+N type diodes and the like in devices. Unfortunately, many other elements used in doping semiconductor materials either have too high a vapor pressure or too slow a rate of migration when employed in TGZM processing.
In particular, the N-type dopants; namely, phosphorus, arsenic, and antimony, have a high vapor pressure at the processing temperatures of 700.degree. C. to 1350.degree. C. normally used for temperature gradient zone melting. Their high vapor pressures cause evaporation of these N-type elements before such elements in the form of an alloy melt with the material of the semiconductor body can penetrate into and encapsulate themselves in the body of semiconductor to prevent subsequent evaporation.
One means of avoiding this evaporation problem associated with N-type dopants is to use a vapor barrier overlay described in our copending application, "Droplet Migration Using a Sealant Layer," Ser. No. 967,281, or to reduce the volatility of the N-type dopants by incorporating them in inert carrier droplets as described in our copending application, "Droplet Migration Using Carrier Droplets," Ser. No. 944,280. In this latter application, an inert carrier metal is used to carry a passenger element that can impart either an N-type or a P-type dopant characteristic to the semiconductor material. The carrier metal is selected so that it penetrates quickly and easily into the semiconductor body, it migrates quickly through the semiconductor body and it does not have a detrimental effect on the electrical properties of the epitaxial semiconductor material deposited behind the migrating droplet during temperature gradient zone melting. The final requirement is that the carrier metal and the passenger element do not react to form a stable compound. For example, aluminum was ruled out as a carrier metal for arsenic because aluminum and arsenic react to form an intermetallic compound Al-As with a reported melting point at 1700.degree. C. Because of the formation of this stable compound, an Al(carrier) - As(passenger) droplet would not migrate but simply react to form the Al-As compound and remain on the surface of the semiconductor body. Similarly, Al(carrier) - P(passenger) droplets were not considered because of the formation of the compound AlP. Likewise, Al(carrier) - Sb(passenger) droplets was ruled out because of the formation of the compound AlSb.
This perceived inability to use aluminum as a carrier metal for the N-type dopants was unfortunate because an aluminum droplet has the best penetration ability of any carrier metal since aluminum can reduce any thin film of silicon oxide on the surface of the semiconductor body and because an aluminum droplet is the fastest migrating droplet of all the carrier metals including gold, tin, gallium, silver and indium.
Consequently, it would be desirable to be able to use aluminum as a carrier metal for at least one N-type dopant in order that N-doping could be accomplished quickly and without surface penetration problems.
An object of this invention, therefore, is to provide a new and improved method of using aluminum as a carrier droplet with an N-type dopant.
Another object of this invention is to provide a new and improved means of producing N-type regions in a body of P-type semiconductor material.
Other objects will, in part, be obvious and will, in part, appear hereinafter.